<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>1比1的div</title>
    <style>
        .parent {
            width: 200px;
            border: 1px solid red;
        }
        .child {
            background-color: green;
            padding-top: 100%;
        }
    </style>
</head>
<body>
    <h4>1比1大小的div</h4>
    <div class="layout">
        <div class="parent">
            <div class="child"></div>
        </div>
    </div>
</body>
</html>